The present invention relates to a golf ball characterized by good flying performance, high scuff resistance, and soft shot feeling.
A two-piece solid golf ball is made up of a core and a cover formed thereon. It is widely accepted by many golfers because of its long flying distance and good durability. On the other hand, it is inferior to a thread-wound golf ball in shot feeling due to stiffness and controllability due to high initial velocity.
There have been proposed several kinds of two-piece solid golf balls with a soft core which are designed to be competitive with thread-wound golf balls in shot feeling.
For example, JP-A 8-276033 discloses a technology to give a solid golf ball having a good shot feeling and a long flying distance. This object is achieved if the golf ball has a specific amount of compressive deformation such that the difference between P and Q is from 1.0 to 3.5 mm, where P denotes the compressive deformation which the core experiences as it receives a changing load from 10 kgf (initial) to 130 kgf (final) and Q denotes the compressive deformation which the ball experiences as it receives a changing load from 10 kgf (initial) to 130 kgf (final).
Also, JP-A 8-294549 discloses a technology to give a solid golf ball that exhibits a soft shot feeling, while keeping the characteristic properties of solid golf balls. This object is achieved if the solid core is soft with an amount of deformation no less than 3.5 mm under a load of 100 kgf and the cover is formed from a soft ionomer resin having a Shore D hardness of 50 to 63 and a 300% modulus of 15 to 35 MPa.
Furthermore, JP-A 2000-245870 discloses a technology to give a golf ball consisting of a core and a cover and having a large number of dimples formed in the ball surface. This golf ball is characterized by soft shot feeling, good spin performance, and good approach controllability owing to the adequate amount of deflection under a load of 100 kgf, the surface hardness of the core, the difference between the hardness of the core surface and the hardness of the core center, and the optimal R/S value, where R denotes the effective contact area which is produced when the face of the driver head comes into contact with the ball at a head speed of 50 m/s and S denotes the virtual contact area of the ball.
However, there is a demand for a golf ball which has a higher rebound resilience and further softer shot feeling so as to comply with golfers"" wish for a longer flying distance.
The present invention was completed in view of the foregoing. It is an object of the present invention to provide a golf ball which exhibits good flying performance, good scuff resistance, and soft shot feeling.
To achieve the above-mentioned object, the present inventors carried out a series of researches, which led to the finding that a two-piece solid golf ball consisting of a solid core and a cover is by far superior to the conventional one in flying performance, scuff resistance, and soft shot feeling, if the core is specified in diameter, flexibility, and raw material (rubber composition), and the cover is specified in thickness, hardness, and raw material (resin composition), so that the golf ball as a whole has flexibility in a specific range.
According to the first aspect of the present invention, there is provided a two-piece solid golf ball made up of a solid core and a cover wherein the solid core is formed from a rubber composition composed of (A) 100 parts by weight of rubber base material containing 60 to 100% by weight of a polybutadiene which is synthesized with a catalyst of rare earth element and contains no less than 60% of cis-1,4-linkage, (B) 0.1 to 0.8 parts by weight of organic peroxide, (C) an unsaturated carboxylic acid and/or a metal salt thereof, (D) an organic sulfur compound, and (E) an inorganic filler and the solid core deforms by 3.0 to 5.5 mm under a load of 980 N (100 kgf) and has a diameter of 37 to 42 mm, and the cover is formed mainly from a mixture of (M) a block copolymer having amino groups at its terminals and (N) an ionomer resin, with the ratio of (M)/(N) being from 3/97 to 60/40 (by weight) and has a thickness of 0.5 to 2.5 mm and a Shore D hardness of 50 to 70, and that the two-piece solid golf ball made up of a solid core and a cover deforms by 3.0 to 5.0 mm under a load of 980 N (100 kgf).
According to the second aspect of the present invention, the polybutadiene may be a modified polybutadiene obtained by synthesis with an Nd-based catalyst as the catalyst of rare earth element and subsequent reaction with a terminal modifier.
According to the third aspect of the present invention, the rubber composition may be one which is composed of (A) 100 parts by weight of rubber base material containing no less than 60% of cis-1,4-linkage and also containing 60 to 100% by weight of polybutadiene synthesized by using a catalyst of rare earth element, (B) more than one kind of organic peroxide compound, (C) 10 to 60 parts by weight of an unsaturated carboxylic acid and/or a metal salt thereof, (D) 0.1 to 5 parts by weight of an organic sulfur compound, and (E) 5 to 80 parts by weight of an inorganic filler.
According to the fourth aspect of the present invention, the cover may have a large number of dimples in the surface thereof such that the dimple volume ratio (VR) may be 0.70 to 1.00% and the dimple surface area ratio (SR) may be 70 to 85%, with VR being defined as the ratio of the sum total of the volumes of individual dimples under the plane surrounded by the periphery of each dimple to the volume of a virtual sphere without dimples in the cover, and SR being defined as the ratio of the sum total of the areas surrounded by the periphery of individual dimples to the surface area of the virtual sphere.
According to the present invention, there is provided a two-piece solid golf ball which is by far superior to conventional ones in flying performance, cover durability, scuff resistance, and shot feeling.